Research Project III Summary Building on extensive past research by Project III on the comorbidity of reading disability (RD) and ADHD, the proposed research will extend those methods and concepts to understanding the comorbidities among less-studied complex LDs, and their comorbidities with ADHD and with RD. Project III will use multiple methods of analysis to examine how component cognitive skills, including executive functions (EFs), contribute to these comorbidities. Previous research has demonstrated that ADHD is comorbid with each of these less-studied LDs, including poor reading comprehension, poor written language, and poor math problem solving ability, and considerable prior research indicates a role for EFs in these disorders. Proficiency in these academic skills requires not only mastery of prerequisite basic skills, but also higher level-thinking skills, such as oral language comprehension and EFs. Project III will explicitly test versions of this interactive model of these three complex LDs in collaboration with Projects II and V. So the goal of Project III is to gain a comprehensive understanding ofthe comorbidities among these three complex LDs and their comorbidities with both ADHD and RD. In collaboration with Project I, Project III will also continue ongoing work on the comorbidity between dyslexia (RD) and ADHD. Project III will test these comorbidities at both the cognitive and etiological levels of analysis. It will also test whether GxE interactions are part ofthe etiology of these disorders and their comorbidities. Even though there is considerable evidence for shared cognitive and genetic risk factors across these disorders, their relations have not been previously examined in the same sample.